Method and device for the continuous detection of the slag level in electroslag remelting installations with short slidable molds

ABSTRACT

A method and an installation for the continuous checking and regulation of the level of the surface of a slag bath in an ESR installation with short slidable molds. For the continuous measurement of the position of the surface of the slag bath, the method employs a radar beam emitted by a radar measuring probe and directed approximately vertically onto the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of AT A807/2010, filed May 14, 2010, and incorporates the same by reference.

BACKGROUND OF THE INVENTION

In electroslag remelting installations (which are also referred to as “ESR installations”) in short slidable molds, it is necessary to keep the level of the slag line, i.e. the level of the molten metal line, approximately constant with respect to the upper edge of the water-cooled mold, in order to avoid thermal overloading of or damage to the installation. Depending on the type of installation, this is achieved by the fact that, depending on the melting rate of an ingot, the re-melted ingot is either continuously or stepwise withdrawn downwards from a mold installed in a stationary position, or the mold is withdrawn upwards in the case of an ingot assembled on a fixed base plate. In both cases, there is a relative movement between the mold wall and the surface of the ingot, in such a way that the ingot is withdrawn downwards out of the mold.

Since it involves a slowly proceeding process in the case of electroslag remelting at the usual remelting rates, with build-up rates of a solidified cast ingot formed from the molten metal of between 10-15 mm/min (in the case of small cast ingots of 300 mm ingot diameter and less) and 1.5-3 mm/min in the case of large ingots with 1000 mm ingot diameter and above, it is possible, especially in the case of open remelting installations, i.e. remelting installations without a cover, to keep the level of the slag line constant with sufficient reliability by means of a visual check with permanently adjusted withdrawal rates readjusted from time to time.

In order to be able to meet the increased quality requirements on the remelting products, electroslag remelting installations are today designed virtually exclusively as closed installations with a protective gas hood, so that the possibility of a simple visual check is no longer possible. Indirect methods for checking the slag line, such as the calculation of the slag line level from the quantity of melted metal of the ingot compared to the withdrawal rate of the cast ingot, are helpful, but they are not sufficient, so that the protective gas hood still has to be raised from time to time in order to be able to carry out an additional visual check. It is in principle possible with this method to check the level of the slag line in an adequate manner, as long as no malfunctions occur in the course of the process. A drawback with this method according to the prior art is that the protective gas effect is disturbed and the entry of air into the space above the slag is enabled, this being undesirable for reasons of casting quality.

Moreover, it is a drawback that malfunctions giving rise to an abrupt change in the slag level cannot be detected in good time by the aforementioned mode of operation. This can for example be a slag or steel breakout, which usually begins slowly, but can then lead to the outflow of the whole slag bath and a part of the metal sump. In this case, there is a rapid fall of the slag line. This can however also occur with a malfunction of the controller which keeps the immersion depth of the electrode in the slag bath constant. In the case of an excessively deep immersion of the electrode, this may lead to an uncontrolled rise in the slag line.

Furthermore, it may also be desired to adjust the level of the slag line differently from ingot to ingot, or to allow the level of the slag line to fluctuate in a targeted manner within certain limits, in order thereby to increase the life of the mold. This is not possible with the checking methods currently available.

SUMMARY OF THE INVENTION

Proceeding from the prior art described above, the object of the invention, therefore, is not only to detect continuously the level of the slag line in open remelting installations, but also in particular in remelting installations closed by a protective gas hood, and, in cooperation with the control devices of the installations and the operating personnel, to initiate automatically or to perform continuously the following functions:

-   -   start of the ingot withdrawal (cast ingot) when a defined         setpoint level of the slag line is reached,     -   checking of the setpoint level of the slag line during the phase         of the ingot withdrawal and control of the ingot withdrawal in         order to adhere to the setpoint level of the slag line,     -   monitoring of the disruption-free remelting process, and     -   control of a desired, controlled fluctuation in the level of the         slag line during the remelting.

A further aim is the immediate detection of malfunctions which cause a comparatively rapid change in the slag level, such as

-   -   breakout of slag and/or metal, associated with a rapid fall in         the level,     -   excessively deep immersion of the electrode during the remelting         or especially after a change of electrode, associated with an         unexpected rise in the level.

It has been shown in tests that the aforementioned aims can best be achieved according to the invention by the use of a radar measuring probe known per se.

The invention therefore relates to a method for the continuous checking of the slag line in ESR installations with short slidable molds with the characterizing features of the continuous measurement of the slag level with respect to the upper edge of the mold by means of a radar probe known per se and installed in the mold cover or the protective gas hood, by a radar beam directed essentially vertically onto the slag surface, and the use of the obtained signal for the automatic control of the ingot withdrawal, in a manner such that the position of the slag line in the mold remains constant or fluctuates in a targeted manner within specific limits.

Furthermore, the mode of procedure according to the invention enables immediate detection of abrupt and distinct changes in the slag line and the triggering of an alarm or the automatic initiation of corrective measures by means of an overriding control system.

In order to carry out the method described above, there is provided according to the invention a radar measuring probe mounted on a mold cover or a protective gas hood, if need be installed in a water-cooled shielding housing, with a measuring tube through which the radar beam is directed in such a way that it strikes the surface of the slag bath approximately vertically. The signal thus obtained, i.e. the slag level, can then be displayed or represented on an instrument or display screen.

In addition, the obtained signal can be used as an input signal for controlling the ingot withdrawal from the mold, or for triggering an alarm in the event of abrupt and distinct changes in the level of the slag line or when specific limits are exceeded or fallen below.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a plan view of an ESR installation;

FIG. 2 is an enlarged view of the housing in FIG. 1; and

FIG. 3 is a vertical cross-section through the housing of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents, in plain view, an example of a possible arrangement of a radar measuring probe 3 disposed in a concealed manner in a water-cooled shielding housing 1, which is flange-mounted on a likewise water-cooled mold cover 2 of an ESR installation 10.

Shown in plain view in a partially cut-away representation in FIG. 2 is the shielding housing 1 flange-mounted on the mold cover 2, with a radar measuring probe 3 disposed in the shielding housing and a bent measuring tube 4 passed above the mold interior, with a purging connection 5 for gas purging and an indicated line 6 for a power supply to the radar measuring probe 3 and for the transmission of measurement signals. It can also be seen that the line 6 is connected to an evaluation device 15, which displays or represents the level of surface 11 ascertained by the radar measuring probe 3. Alternatively, or in addition thereto, the line 6 can also be connected to a control device 20 of the ESR installation 10. The control device 20 either keeps the level of surface 11 of the slag bath 9 constant or allows it to fluctuate periodically in a targeted manner within predetermined limits. Furthermore, the control device 20 can be connected to an alarm device 21, which triggers an alarm when the limits for the level of the surface 11 are exceeded or fallen below.

FIG. 3 shows a vertical cross-section through the shielding housing 1 with the radar measuring probe 3 and the bent measuring tube 4, which transforms into a conically shaped horn antenna 7. It is also possible to see an inner wall 8 of a water-cooled slidable mold 13 as well as the surface 11 of the slag bath 9 (metal line). Also indicated is a radar beam 12, which is generated by the radar measuring probe 3 and which preferably strikes the surface 11 at least approximately vertically.

In the illustrated embodiment, the shielding housing 1 is designed as a double-shell housing and enables water-cooling. In addition, the shielding housing 1 can also optionally be provided with an mu-metal layer in order to shield against electromagnetic fields. An mu-metal layer is understood to mean a soft-metal alloy which has a relatively high permeability. In particular, the mu-metal layer can contain approx. 75-80% nickel, 15% iron and approximately 3-4% copper, cobalt and/or molybdenum.

While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principle. 

1. A method for electroslag remelting in a short slidable mold, wherein a level of a surface of a slag bath in an electroslag remelting installation is monitored, the method comprising the steps of: measuring the level of the surface of the slag bath with a radar measuring probe; directing a radar beam generated by the radar measuring probe so that the beam strikes the surface of the slag bath; and at least one of displaying the ascertained level of the surface by way of an evaluation device and using the ascertained level as an input signal for a control device for an ingot withdrawal from the slidable mold, so that the level of the surface of the slag bath is either kept constant or fluctuates periodically in a targeted manner within specific limits during the operation of the electroslag remelting installation.
 2. The method according to claim 1, wherein the radar measuring probe is disposed in a mold cover or a protective gas hood of the remelting installation.
 3. The method according to claim 1, including continuously measuring the level of the surface of the slag bath.
 4. The method according to claim 1, wherein the radar beam strikes the surface of the slag bath substantially vertically.
 5. The method according to claim 1, further including triggering an alarm in the event of abrupt, distinct changes in the level of the surface of the slag bath or when specific established limits of the level of the surface of the slag bath are exceeded or fallen below.
 6. An electroslag remelting installation for carrying out the method according to claim 1, comprising: a mold cover or a protective gas hood for covering a slag bath; and a radar measuring probe disposed on the mold cover or the protective gas hood for detecting a level of a surface of the slag bath, the probe generating a radar beam that strikes the surface of the slag bath.
 7. The electroslag remelting installation according to claim 6, and further comprising a measuring tube attached to the radar measuring probe so as to deflect the radar beam so that the radar beam strikes the surface of the slag bath substantially vertically.
 8. The electroslag remelting installation according to claim 6, and further comprising an evaluation device that displays or represents the level of the surface of the slag bath.
 9. The electroslag remelting installation according to claim 6, and further comprising a control device arranged to receive a signal from the radar measuring probe, the control device being operative to use the signal as an input signal for ingot withdrawal from the slidable mold.
 10. The electroslag remelting installation according to claim 9, and further comprising an alarm device in communication with the control device, wherein the control device is operative to activate the alarm device in the event of abrupt and distinct changes in the level of the surface or in the event of specific preset limits of the level of the surface are exceeded or fallen below. 